Oaks ‘n’ Folks – Volume 9, Issue 1 – April, 1994
Examination of factors suspected of contributing to poor seedling regeneration of blue oak (Quercus douglasii) and valley oak (Q. lobata) was initiated in 1985 at 6 locations statewide, including the Hopland Research and Extension Center in Mendocino County.
Figure 1. Five-season survival (% of acorns seeded) of blue oak growing with and without control of herbaceous plants and with and without screen protection in 2 seedlings: CYNRN 1987. HREC 1986. SFREC
1987. Observations made in spring after bud break.
Figure 2. Five-season survival (% of acorns seeded) of valley oak growing with and without control of herbaceous plants and with and without screen protection in 2 seedlings: BRP 1987. HREC 1986, 1987. Observations made in spring after bud break.
The most important factor for reduction of moisture stress in seedling experiments has been weed control. Average emergence in all blue oak seedlings with and without weed control was 45% and 29%, respectively. The respective values for all valley oak seedlings were 60% and 46%. Average first year survival, expressed as a percentage of acorns planted, was improved with weed control in seedings of both blue oak (34% vs. 12%) and valley oak (45% vs. 29%). Limited data suggests the differential in survival increases over time as overall survival declines (figs. 1 and 2). With few exceptions, the addition of screen protection to discourage predation significantly enhanced survival and growth (figs 1 and 2, table 1). Shade provided by window screen cages is suspected of making an unmeasured positive contribution.
Interaction between weed control and protection develops over time.
Table 1. Average effects of herb control and screen protection on 1992 seedling height (cm) in 5 seedlings of blue oak. Hopland Research and Extension Center (HREC) and Sierra Foothill Research and Extension Center (SFREC), 1986; Canyon Ranch and SFREC, 1987; HREC 1988; and 4 of valley oak HFS, 1986, Briones Regional Park and HFS, 1987; HFS 1988.
Table 2. Average survival (%) through 1992 pf 5-, 4-, and 3-year-old blue oak nursery stock planted with 4 weed control strategies in 1988, 1989 and 1990 at the Hopland and Sierra Foothill Research and Extension Centers.
At both the Hopland and Sierra Research and Extension Centers, three successive blue oak plantings were established in winter 1987-88, 1988-89, and 1989-90 using 2- to 3-month-old nursery stock for the evaluation of alternative competition control strategies. Three strategies (a one-time herbicide application and 2 synthetic mulch mats) are being compared to no weed control. Measured over time, blue oak seedling survival is significantly improved by any form of weed suppression compared with no weed control (table 2), but data collected in 1993 show no differences among strategies. Weed control nearly doubled survival in the group planted in 1990 (3 years old) and promises to make the difference between survival and no survival in the 1988 group (5 years old).
Of note is the lower survival among strategies applied to 1990 seedlings compared to 1989 seedlings (4 years old). When compared to no weed control, this differential was significant for the impervious polyethylene (IP) treatment (table 2; only two-thirds as many seedlings survived in 1990 planting.
In 1992, over 80% of all mortality recorded at Hopland in spring was caused by rodents. Of this voles were responsible for about two-thirds. The remainder, or about one-quarter of all mortality, was attributed to gophers. The IP weed control treatment appeared to aggravate the problem by providing cover for rodents and exposing the younger seedlings to greater damage. A contributing factor to rodent damage in the weed control strategy study was absence of all weed control (with the exception of treatments applied) for 3 to 5 years within the half acre exclosure used. The accumulation of biomass created habitat that apparently attracted rodents, particularly voles, during an increase in natural populations.
A positive aspect of the use of mulch mats is the influence on growth (table 3). Average 1992 height of Hopland seedlings protected by plastic mats applied in 1989 and 1990 was 35 cm and 26 cm, respectively. These values are different from both no weed control and herbicide. The difference may be a product of weed suppression over time and the influence of mats on the soil environment.
Table 3. Average 1992 height (cm) of 5-, 4-, and 3-year-old blue oak seedlings developing from 2-month-old nursery stock planted with 4 weed control strategies at the Hopland Research and Extension Center in 3 consecutive years: 1988, 1989, and 1990.
It is interesting to note that a single application of herbicide at plantings is as effective for promoting survival as any other strategy being tested. Although the influence of this treatment on growth is not equal to that of the best mulch mat treatment, it may be adequate in most situations. From the standpoint of cost effectiveness, a single application of herbicide at planting may be the most practical treatment when nursery stock is used in restoration in projects.
Initiated in 1988, was a direct comparison between restocking techniques (directly seeded acorns and 2- to 3-month-old nursery stocks) that may be useful on rangelands. Forty small 6′ x 6′ exclosures per acre were established on one-acre sites at Hopland (one site each for blue and valley oak) and on one-acre sites at the Sierra Foothill Research and Extension Center (blue oak) and in Monterey County (one site each for blue and valley oak). Within each exclosure, acorns and transplants were planted with weed control, and half of each plant material was protected with screens. A total of 4 treatments was used, and each included either 3 acorns or 3 transplants.
Survival of seedlings developing from transplants is greater at all but one location (table 4). At the Harden Ranch in Monterey County, valley oak transplants were attacked by ground squirrels immediately after planting. Control of the rodents prevented total loss, but survival of seedlings developing from the 2 classes of plant material cannot be fairly compared. At Hopland, survival of valley oak transplants is greater, but the difference is small, less than 10%. By comparison, average survival of blue oak transplants at 3 sites is 40% greater than survival of seedlings developing from directly planted acorns.
Table 4. Fourth year seedling survival (percent of planted spots occupied) in 4 treatments of blue oak (3 sites: Hopland Research and Extension Center (HREC); Sierra Foothill Research and Extension Center (SFREC); Work Ranch (WORKRN), Monterey County, and valley oak (2 sites: HREC; Harden Ranch (HRDRN), Monterey County) developing from seeded acorns and 2-month-old nursery stock planted in the 1988-89 season. Survival measured in spring 1993 after bud break.
Average survival of protected blue oak seedlings is nearly 3 times greater than that of unprotected seedlings (table 4). However, protection has had no significant influence on survival of valley oak (table 4).
Protection, but not class of plant material, has favorably influenced growth in all 5 plantings (table 5). Average height of protected blue oak seedlings is 3 times greater than that of unprotected seedlings. Protected valley oak seedlings are 40% taller.
Table 5. Fourth year seedling height (cm) in 4 treatments of blue oak (3 sites: Hopland Research and Extension Center (HREC); Sierra Foothill Research and Extension and Center (SFREC); Work Ranch (WORKRN), Monterey County) and valley oak (2 sites: HREC; Harden Ranch (HRDRN), Monterey County) developing from seeded acorns and 2-month-old nursery stock planted in the 1988-89 season. Measurements made in fall 1992.
Use of 2- to 3-month-old nursery stock provides higher survival compared with directly seeded acorns, at least in plantings of blue oak. The value of screen protection to enhance survival of blue oak in these plantings has been clearly demonstrated; protection is recommended for blue oak plantings on rangeland, without qualification.
Based on the studies discussed in this review, weed control and protection against insect and rodent herbivory are recommended in all oak plantings.
Data being collected in studies reviewed will help define survival curves that can be used in planning oak restoration and mitigation.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc